1. Technical Field
The present invention generally relates to printers and, more particularly, to inkjet printers in which aqueous ink is applied to a porous sheet medium such as paper.
2. Background Art
Conventional inkjet printers include inking devices, generally referred to as "pens," for depositing ink droplets on sheets to be printed. Normally, the droplets contain an aqueous fraction which, after printing, must be evaporated to permanently fix the ink to the printed sheets. With the increased use of highly aqueous inks, many having water contents approaching one-hundred percent by weight, several printing problems have arisen. One such problem is that highly aqueous inks cause wetted fibers on the printed face of a sheet to swell to a substantially greater extent than dry fibers on the obverse side of the sheet. Such an effect, often described as differential expansion, results in wrinkle-like bulges, or cockles, in sheets. When printing on ordinary paper, cockling can occur as rapidly as 600 milliseconds (ms) after aqueous ink is applied.
Also, highly aqueous inks cause difficulties in sheet drying. Conventionally, the drying of ink on printed sheets entails applying heat after entire sheets are printed. This practice has several disadvantages in the case of highly aqueous inks. For instance, in the interval while a printed sheet is transported from a printing station to a drying station, highly aqueous inks are quite susceptible to smearing. Also, highly aqueous inks often bleed into paper fibers before drying is complete. Such bleeding can detrimentally affect the appearance of text or graphics printed on a sheet and, also, can adversely affect the appearance of the obverse side of a printed sheet.
The highly aqueous nature of many modern inks can also adversely affect the efficiency of inkjet printers. For example, to provide adequate time for highly aqueous inks to dry, the printing speed of an inkjet printer may have to be slowed or else the size of the driers on the printer may have to be increased. Although the temperature of driers can be increased to dry ink more quickly, there are limits beyond which temperature cannot be elevated without scorching printed sheets.
In addition to the problems mentioned above, there are less obvious ways in which highly aqueous inks may adversely affect inkjet printing. For example, because inkjet printing normally proceeds sequentially from location to location across a sheet surface, cockling at one location can adversely affect pen-to-sheet spacing during printing at adjacent locations. Pen-to-sheet spacing is especially critical in bi-directional inkjet printing (i.e., in inkjet printers that print swaths of ink drops while moving both from right-to-left and from left-to-right across the surface of a sheet). In bi-directional printing, print defects are usually perceptible unless pen-to-sheet spacing distance is held constant to tolerances of about .+-.0.0025 inch.
In view of the preceding discussion, it can be appreciated that there exists a need in the inkjet printing art for improved ways and means to minimize cockling and to prevent highly aqueous inks from bleeding and smearing before drying.